Wireless wearable communications are fields of increasing research interest due to the numerous potentials offered in areas such as healthcare and fitness monitoring, mobile network/internet, smart skin and functional clothes to name a few. Radio frequency (RF) transceiver is a basic building block in any communications system, which receives RF signal and converts it to lower intermediate frequency (IF) so that the signal can be readily for analog to digital conversion (ADC) and digital signal process (DSP). A RF transceiver includes passive components such as antennas, transmission lines (TLs) and impedance matching networks and active circuits such as low-noise amplifier (LNA), frequency mixer and local oscillator to name a few. Conventionally, a RF transceiver is mainly fabricated with PCB (printed circuit board) assembly technology, which poses a big challenge in integration with flexible substrates like papers and textiles. To tackle this, researchers have proposed techniques of coating/plating metal on textile yarns, dyeing carbon nanotube on textile then sputtering with Ag/Au particles to make wearable antennas. However, these approaches, even though the metals were deposited on textile substrates, the fabrication procedures and materials used were complex and expensive, not suitable for mass deployment for low cost wireless wearable applications.
Graphene, single layer of carbon atoms arranged in a hexagonal lattice, is a very promising material for wireless wearable communications applications owing to its unique electronic and physical properties. To date, researchers have intensively explored the applications of graphene to make active devices such as transistors and diodes. A quaternary digital modulator was achieved using two graphene transistors. Amplifiers at RF bands were demonstrated experimentally with graphene field-effect transistors. Other active devices that are essential in a RF transceivers such as frequency mixer and oscillator were also demonstrated. More recently monolithic graphene RF receiver integrated circuit (IC) performing signal amplification, filtering and down-conversion has also been reported.
However, even though profound progress has been made in graphene active devices, the pace of developing graphene passive RF components has far lagged behind. This is because, in spite of graphene's high conductivity, both exfoliated and CVD (chemical vapor deposition) graphene sheets have very high surface resistance, hindering their applications in RF passive components. However, recent development of graphene conductive ink has brought the possibility along with its superiority in high conductivity, mechanical flexibility, light weight and low cost. Preparation of graphene conductive inks can be generally categorized into two groups. One is binder-free technique which disperses the graphene directly in solvents such as N-Methyl-2-pyrrolidone or Dimethylformamide (NMP/DMF) without adding any binder, whereas the other uses binders like ethyl cellulose (EC). Even though the latter technique can offer higher conductivity, it requires high-temperature thermal annealing, making it incompatible with heat-sensitive substrates like papers and textiles. On the other hand, binder-free technique is compatible with heat-sensitive substrates thanks to its low temperature annealing, however much further improvement of ink conductivity is required for RF applications.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.